In the process of injection molding the transformation of a polymeric material, typically thermoplastic such as PET in the form of granules, into a manufactured product with defined shape occurs. The process begins with the high pressure injection of the molten material into a molding cavity generally having walls made of steel, and ends with the extraction of the preform from the mold after solidification by cooling. The cooling modes and time are critical factors in the molding process. By cooling time it is meant the time in which the component is inside the mold, but there is no more flow of molten material. Actually, the physical phenomenon of the cooling of the molten material begins as soon as this is injected into the cavity and comes into contact with the cold walls of the mold that, as said, are generally made of steel. This phenomenon of rapid cooling can lead to the cooling of the molten mass, or of a part thereof, before it can reach the mold surface. For the complete filling of the mold to occur, the injection speed and pressure must be high enough to counteract the phenomenon of rapid cooling of the molten mass. For molding PET preforms with thin walls, higher performance of the machine is required as in this case the problem of rapid cooling is more accentuated, moreover the consequences of the friction of the molten mass with the steel walls of the mold can be more significant. The consequence is the potential risk of getting deformed or incomplete preforms (short shot). It is known that in the PET container industry, injection molding of preforms which have a total length L greater than or equal to 100 mm and an L/t ratio greater than 50, where “t” is the thickness of the wall, is particularly delicate for the above reasons. Molding preforms with L/t ratio greater than 50 using traditional equipment, molds and materials results in technical difficulties hard to overcome. In fact, in order to counteract the phenomena of freezing of the material during injection, high injection speed and high pressures would be required. The latter, in particular, would generate strong forces on the elements forming the mold and which should be contrasted to prevent the undesired opening thereof, by necessary additional strong closing force of the press platens.
All this general increase of the forces involved would lead to a high wear, if not even to the failure, of the elements forming the mold and of the elements forming the injection press.
Apart from the above-described technical problems, in any case there would not be the certainty of obtaining a correct molding of the preforms. In fact, high molding pressures and high speed of molten material would result, the first ones, in an occurrence of burrs (flash) on the preforms and, the second ones, in the difficulty of expulsion of air and gas from the molding cavity during the operation of filling by the molten material. This second phenomenon would, once again, lead to an incomplete formation of some parts of the preform and in particular the neck end surface, the crest of the threads and of the grip ring.
Moreover, the parameters related to the molding of preforms with thin walls have necessarily very narrow process windows (requiring very small suitable temperature and pressure ranges) and therefore the risk of incomplete forms (short shot) increases and so the number of rejects. The same problems can already be encountered for preforms having a length lower than 100 mm having an L/t ratio greater than 45. In order to improve and facilitate the filling and the ability to inject preforms with an L/t ratio of between 60 and 65, an expedient might be to increase the temperature of the molten PET during the injection step so that PET remains more fluid, considering that the viscosity of the polymer is highly dependent on its temperature during the process. This expedient can help, but its main drawback is the negative impact that the increase in temperature has on the duration of the molding cycle, since the higher the temperature of the molten PET, the longer the duration of the cycle, due to the longer cooling time, and therefore the lower the productivity of the system. Another drawback consists, under these conditions, in the drastic but undesired increase in the level of acetaldehyde and this is an additional problem for the production of containers for beverages, especially those beverages, such as water, for which the quality of taste is a critical factor. Another well known expedient in the industry is to modify the finish of the steel surface to reduce the mechanical interaction between steel and PET. This can be achieved by applying a surface finish such as that reproducing the surface of lotus leaves which is known for its anti-adhesive and self-cleaning properties. To date, however, no solutions are known which can improve the production of preforms with thin walls, and both for preforms with L lower than 100 mm and thin walls having the L/t ratio>45, and for preforms with L≧100 mm and thin walls having L/t>50, there is a technological limit, as previously disclosed. A further drawback of the traditional molds consists in the difficulty of opening and removing the preform from the mold (demolding). The need of finding a solution to the problem is therefore felt.